Mass burning speed and onset of auto-ignition of premixed syngas/air and gas to liquid (GTL)/air mixtures respectively, have been determined at high temperatures and pressures over a wide range of fuel-air equivalence ratios. The experimental facilities consist of two spherical and cylindrical vessels. The spherical vessel was used to collect pressure data to measure the burning speed, mass burning rate and determine the onset of auto-ignition and cylindrical vessel was used to take pictures of flame propagation with a high speed CMOS camera up to 40,000 frame per second located in a Schlieren system. Effect of cellular flames on mass burning rate have been determined. Critical pressures and temperature for different fuel air equivalence ratios at which auto-ignition takes place have been measured. In this paper, mass burning rate of syngas is calculated for a wide range of equivalence ratio from 0.6 to 2, unburned temperature from 400 to 680 K and initial pressure from 2 to 25 atm. A power law correlation has been developed as a function of equivalence ratio, temperature and pressure. The onset of auto-ignition for GTL/air mixture has been identified for equivalence ratio of 0.8 to 1.2.